Phosphate accumulating organisms (PAOs) perform a storage polymer metabolism within an anaerobic-aerobic cycle. Anaerobically, PAOs take up volatile fatty acids (VFA) and store them as poly-ẞ-hydroxyalkanoates (PHA). The energy (mainly ATP) necessary for the transport and storage of VFA (and general maintenance) is obtained through the cleavage of polyphosphate. While the reducing equivalents (e.g. NADH) for VFA storage are obtained through the cleavage of glycogen and/or from the anaerobic operation of the TCA cycle. Aerobically, PAOs replenish their reserves of polyphosphate and glycogen, resulting in P uptake, whilst degrading PHA to obtain a carbon and energy supply for growth.
PAOs have the metabolic flexibility to adapt the synthesis of each polymer according to the resources available in the environment, and thus affecting the growth of the organism. Hence, within a PAOs metabolism, there is a trade-off between the use of glycogen and polyphosphate. This trade-off is dependent on the cell's requirements to obtain ATP and NADH for PHA storage. In turn, ATP and NADH amounts can be obtained in different ratios depending on the active metabolic routes. This thesis aims to determine what is the mechanism controlling this trade-off and if there are limits to this relationship.
To investigate this trade-off, a metabolic model for PAOs was created and simulated through a conditional flux balance analysis (cFBA) approach. The resulting amounts of the metabolites simulated with this approach were comparable to those obtained experimentally (figure 6). Additionally, this model was simulated to different sets of starting amounts of glycogen and polyphosphate at a constant acetate feed of 3.84 mCmol/gdw, and the resulting growth was compared between each simulation. This led to an optimal range of initial polyphosphate amounts [2.1-23.5 mPmol/gdw] and initial glycogen amounts [0.3-1 mCmol/gdw]. In reality, these glycogen amounts were never observed experimentally and to the extent of our knowledge never have been reported in PAOs literature. This suggests a glycogen minimal limit amount (e.g. 1 mCmol/gdw), that might reveal a robustness mechanism employed by PAOs to guarantee survivability in uncertain environments. In parallel, a thermodynamic analysis was performed on the malate dehydrogenase reaction, which led to the conclusion that this reaction is not feasible in an anaerobic environment, potentially highlighting a control mechanism.